Directional borehole operations, such as directional drilling, involve varying or controlling the direction of a downhole tool (e.g., a drill bit) in a wellbore to direct the tool towards a desired target destination. In directional drilling, for example, the direction of a drill bit is controlled to direct the bit, and the resultant wellbore, towards a desired target destination.
Various techniques have been used for adjusting the direction of a tool string in a borehole. Slide drilling employs a downhole motor and a bent housing to deflect the borehole. In slide drilling, the direction of the borehole is changed by using the downhole motor to rotate the bit while drill string rotation is halted and the bent housing is oriented to deflect the bit in the desired direction. Slide drilling systems are subject to various problems related to halting drill string rotation. For example, a non-rotating drill string is subject to sticking in the wellbore.
In contrast to slide drilling systems, rotary steerable systems allow the entire drill string to rotate while changing the direction of the borehole. By maintaining drill string rotation, rotary steerable systems overcome various deficiencies of slide drilling. A tool for controlling deflection in a rotary steerable system (i.e. a rotary steerable tool) generally includes a shaft that rotates with the drill string, and a housing surrounding the shaft that includes a device that applies a bending force to the shaft. By bending the shaft, the direction of the downhole end of the shaft is changed.
Directional systems require information to orient the downhole tool toward the desired destination. A slide drilling system must determine the orientation of the bent housing, while a rotary steerable system must determine the orientation of the housing surrounding the shaft. Consequently, the downhole tool generally includes one or more sensors that provide tool orientation information to a control system. The control system uses the orientation information to steer the tool. Some steerable tools include a magnetic compass usually of the electronic magnetometer variety to provide tool orientation information. Accelerometers can also be used to aid the compass in determining the horizontal component of the magnetic field to better calculate the North direction, especially in hole angles above 5° of inclination.